When the surfaces of two bodies are brought into juxtaposition with one another under static conditions, the contact between the two surfaces does not exist over the entire area. This is because, on an atomistic basis, all surfaces, no matter how flat and no matter how highly polished, have asperities. For example, when one of the bodies is statically placed on top of the other, there are a number of asperity contacts, and the asperities are elastically deformed so that the contact area involved in bearing the load equals the unit strength of the material of the load-bearing body. If the load increases, the asperities are further elastically deformed and other lower level asperities are contacted and deformed. The deformation of the asperities will remain within the elastic limits of the material of the load-bearing body unless the load increases to an extent that the deformation extends beyond the elastic limits of the material.
Now, should the two bodies be moved relative to one another in a sliding contact, there will be a continual redistribution of the loading between the asperities, with the contact area always being dynamically equal to the unit load-bearing capacity of the material forming each of the two bodies. In time, as the two contacting surfaces wear into one another, plastic deformation of the asperities will occur and the two surfaces will become smoother. During the wear-in process, however, the asperities are sheared to some extent, and a certain amount of debris is generated. In the absence of a lubricant, the debris is smeared across the contacting surfaces and micro-fusion often occurs so that the contacting surfaces are continuously subjected to a welding and to a shearing action, and the interface rapidly increases in temperature causing the contacting surfaces quickly to deteriorate and fail.
When a lubricant is used, the generated debris is carried by the lubricant. Since the lubricant is nearly incompressible, one of its functions is to serve as a load-bearing medium which distributes the load over large areas of the contacting surfaces and thereby inhibits drastic asperity contacts. The lubricant also serves as a cooling agent for the micro-fusion action.
The usual prior art practice in the case of two contacting bodies which are to engage one another in a rolling or sliding contact is to form the two bodies of a tough material. However, tough materials have high friction coefficients because their contact energy is high. The solution is this dilemma from earliest times has been to introduce lubricants into the contact area. Hard materials, on the other hand, have low friction coefficients and have very high melting points, and are more suited than tough materials for providing sliding or rolling contact surfaces. However, hard materials are universally brittle, and as such cannot in and of themselves absorb much impact energy.
The process of the present invention makes use of the superior properties of hard materials as bearing surfaces by providing a micro-thin film of a hard material on a substrate formed of softer and preferably tougher material. In the process of the invention, the hard material film is made sufficiently thin so that it cannot establish its own identifiable modulus of rigidity, and so that essentially all impact energy is transmitted through the micro-thin-film of hard material to the tough substrate which is capable of absorbing it. Thus, in the practice of the invention, the hard material film serves its function of providing a low friction contact surface, and the high friction tough material of the substrate is held out of engagement with the contacting surfaces, but serves to absorb all impact energy therefrom. The hard material film is preferably prepared with sufficient defects to obviate the formation of grain boundaries therein, for reasons to be explained.
The provision of the micro-thin-film of hard material on the softer substrate in accordance with the process of the invention serves to maintain the surface film intact, even though plastic deformation may occur of the asperities of the substrate below the overlying hard surface film. By this technique, the surface does not have any tendency to generate debris or increase friction coefficients through micro-welding. The micro-thin-film serves to peen down the underlying substratum surface, so that the friction coefficient is actually reduced because the elastic energy required in continual asperity deformation is minimized. With the extremely smooth surface provided by the hard material micro-thin-film in accordance with the process of the invention, the unit load borne by the surface is reduced because the total load is distributed over a larger region than that which is just required to support the load, as would be the case without the film. Plastic deformation now occurs within the interior of the substrate body when the film is present, rather than on the surface of the body. This internal plastic deformation will be referred to herein as "endoplastic deformation".
In summary, the present invention is concerned with a process by which a hard material is intimately bonded to a substrate of a softer and preferably tougher material as a film of micro-dimensions, such that any impact energy is absorbed by the substrate and not by the film itself. The film, therefore, remains intact and is fully compliant with the substrate as the asperities of the substrate are reduced in size by the load. The film is maintained thin enough so that it does not establish its own modulus of rigidity, and so that it exhibits an unusual amount of elastic displacement capacity. In addition, a high degree of disorder is maintained in the hard surface film so that grain boundary failure does not exist.